Atlantoaxial instability is a condition in which the range of motion between the atlantoaxial and pivotal spine (first and second cervical vertebrae) is for some reason outside of normal limits and causes clinical manifestations such as occipital and cervical pain and discomfort and/or limb numbness and weakness. Atlantoaxial instability can be caused by trauma, septic infection, rheumatoid, tumor, etc. The diagnostic criteria are that the distance between the posterior edge of the anterior atlantoaxial arch and the anterior edge of the dentate process of the cardinal vertebrae (atlanto-anterior gap) exceeds 3 mm in adults and 5 mm in children. less severe symptoms may consist of occipital and cervical pain and discomfort, while more severe symptoms may include numbness and weakness of the extremities, limitation of motion, or even difficulty breathing. Patients with severe atlantoaxial instability require surgical treatment, of which posterior atlantoaxial fusion is the most widely used procedure. Its indications include: 1) persistent aggravation of atlantoaxial instability; 2) persistent pain and discomfort in the occipital neck, for which conservative treatment is ineffective; 3) significant cervical spinal cord compression symptoms (numbness and weakness of the limbs, etc.). If there is a clear indication for surgery, early surgery is recommended to avoid liquefaction of the high cervical spinal cord due to long-term compression (clear high signal in the spinal cord on the T2-weighted image of MRI), resulting in slow recovery of neurological function, or severe damage to the high cervical spinal cord resulting in paralysis or even life-threatening injury due to minor trauma causing violent activity of the previously unstable atlantoaxial spine. Typical case Mr. X, a rehabilitation physician in a hospital in P, J, had a neck injury 35 years ago (at the age of 7), when he had neck pain for a long time. 10 years ago, he had numbness in his left hand, which disappeared on its own after a few days. 2 weeks ago, he developed inflexibility in both hands without any obvious cause, and his right hand was heavy. A cervical MRI was performed at a local hospital and abnormal signals were found in the high cervical spinal cord. After coming to our hospital, a cervical hyperflexion and hyperextension examination revealed atlantoaxial instability, which was consistent with the above-mentioned surgical indications. After perfecting the preoperative preparation, a posterior atlantoaxial fusion was performed, and because of the difficulty in placing the pedicle screw in the left side of the patient’s cardinal spine, the left cardinal vertebral plate hook was used for fixation. Three months after the operation, the patient’s neurological symptoms were well relieved but not fully recovered. The iliac implant block had fused with the atlantoaxial spine, and the abnormal signal in the high cervical spinal cord did not expand further. High signal within the spinal cord is an imaging manifestation of substantial damage to the spinal cord and often predicts slow recovery of neurological function after surgery. Therefore, the timing of surgery for atlantoaxial instability is best before substantial damage occurs in the high cervical spinal cord to avoid poor neurological recovery due to surgery after the appearance of intra-spinal high signal on MRI. A and B: preoperative cervical hyperflexion and hyperextension radiographs showing atlantoaxial instability; C: preoperative cervical MRI showing high signal within the high cervical spinal cord as an imaging manifestation of substantial damage to the high cervical spinal cord; D: posterior atlantoaxial fusion postoperative CT 3D reconstruction; E and F: 3 months postoperative cervical frontal and lateral radiographs; G: 3 months postoperative cervical CT sagittal reconstruction showing that the iliac implant block has fused with the atlantoaxial spine; H. MRI of the cervical spine 3 months postoperatively showed no further expansion of abnormal signals within the high cervical spinal cord.